1. Field of the Invention
This invention relates to charging equipment for charging a secondary battery such as a lead battery.
2. Description of Related Art
In a case where a secondary battery such as a lead battery is charged using this type of charging equipment (hereafter, a compact type of charging equipment is particularly referred to as "charger"), a constant-voltage and constant-current charging operation as shown in FIGS. 1 and 3 has been generally carried out. FIG. 1 shows the relationship between a charging current and an output voltage (an output characteristic of a charger) in such a charging equipment, and a desired charging is carried out by properly using a constant-voltage region and a constant-current region. FIG. 2 shows the relationship (charging characteristic curve) between charging voltage/current and a charging time, and the charging of this case is also carried out by properly using a constant-voltage region and a constant-current region.
As this type of charging equipment for performing the charging in the manner as described above has been known a charging equipment as disclosed in Japanese Laid-open Patent Application No. 3-82343, for example. In this charging equipment, an over-charging state of a secondary battery is detected and a charging current is interrupted or switched to a low-level charging current. Through this operation, the over-charging of the secondary battery is prevented, and life-time, safety, etc. of the secondary battery can be assured.
Another type of conventional charging equipment which is disclosed in Japanese Laid-open Patent Application No. 1-194828 has been also known. In this charging equipment, a battery (secondary battery) is rapidly charged in accordance with a feedback signal from an internal resistance free electrochemical potential.
In any charging equipment, detection of termination (completion) of a charging is required for performing the charging. For example, the following two methods have been adopted as charging-termination detecting methods.
(I) A method for beforehand setting a charging time by a timer and detecting the termination of the charging on the basis of the elapsed charging time.
In this method, as shown in FIG. 3A, the charging time is beforehand set by the timer, and when the set time comes, the charging to a secondary battery is terminated irrespective of its residual charge capacity (insufficient charge capacity) on the basis of the judgment that the predetermined charging time elapses.
(II) A method for detecting a predetermined voltage or current value, and terminating the charging on the basis of the detected voltage or current value.
In this method, as shown in FIG. 3B, when the charging voltage or current reaches a predetermined value, the charging is terminated on the basis of the judgment that a secondary battery is fully charged.
However, the former method (I) has a problem that the charging to a battery which has been already fully charged can not be terminated unless a predetermined time elapses, and thus there occurs dispersion in charge capacity. In addition, this method (I) has also a problem that the timer setting is required to be altered, for example when a high-capacity battery having plural batteries (for example, two batteries) arranged in parallel is used, and this work is cumbersome.
On the other hand, the latter method (II) requires the set voltage or current value to be adjusted with high accuracy. Therefore, this method has a problem that a circuit construction is complicated and a cost is heightened. In addition, a voltage drop of the charging current occurs due to variation in internal impedance of a battery or by a contact resistance between a battery terminal and a charger, so that dispersion in charge capacity occurs due to difference in contact resistance between batteries which have been already charged.